The government can disagree with your return based on their data without auditing. A couple of years ago, the government told me they thought I'd under-reported my taxable income from a couple of years earlier (I had, because a place I'd worked had failed to give me a 1099). I wasn't required to undergo an audit; they just said they thought I owed this much money and I should either pay it or explain why I didn't actually owe it.